The comparison scores every group at once. This page is for the moment you already have a pair of names in mind — mayfly or stonefly, scud or sowbug, shad or alewife — and want the structures that keep them apart, in one look. Nothing here is a current hatch.
The differences, and nothing else
Best separating check: How the wings restAt a glance It separates Hellgrammite from Stonefly cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Hellgrammite
Stonefly
How the wings restdecisiveAt a glance
Varies
Wings held flat and parallel along the body
Tail filamentsdecisiveClose look
Varies
Two tail filaments clearly visible
Two long antennaedecisiveClose look
Not diagnostic
Yes
Gills at the thorax or leg basesdecisiveHand lens
Not diagnostic
Yes
Terminal prolegs or hooks at the rearClose look
Yes
Not diagnostic
Reported size
25–100 mm
5–75 mm
Water it is recorded in
river, stream
river, stream, spring
Life stages covered
larva
nymph, adult
How it moves
strong crawling on rock; mostly crawls, but can swim by undulating the body
crawling on rock or debris; weak flight; adults often stay near the ground
Reference seasonality
Largest larvae leave the water: May, Jun
Pre-emergence migration: Apr, May, Jun, Jul · Adult activity: May, Jun, Jul
Rows come from the reviewed diagnostic weights the comparison scores with. “Decisive” means one answer puts them at least four points apart — the carrier's own rule for a structure that separates a pair.
The two entries, side by side
Megaloptera · Corydalidae
Hellgrammite
Big, armoured, and genuinely able to pinch you back.
First check
Size first. If it is under 25 mm you are almost certainly not looking at a mature hellgrammite. Then check for the row of lateral filaments down both sides.
What it is
The hellgrammite is the aquatic larva of a dobsonfly, and it is the largest thing most people will turn over a rock and find. Mature larvae run about 50–90 mm and reach about 100 mm. Nothing else in cold flowing water looks quite like it once you have seen one.
Stages
Larva (25–100 mm (mature larvae mostly 50–90 mm))
Water
river, stream
Separates from Stonefly
Stonefly nymphs have two long tail filaments and no row of lateral abdominal filaments. Hellgrammites have the filament row and no long cerci.
Typical windows
Larvae present (larva) · Largest larvae leave the water (larva)
Patterns (correspondence)
Hellgrammite / Woolly Bugger
Limits
Hellgrammite refers specifically to the aquatic larva of dobsonflies (Corydalidae); related alderfly larvae (Sialidae) are smaller and differ in filament arrangement.
Two tails, two antennae, and a poor opinion of swimming.
First check
Two tails AND two long antennae, with any gills at the thorax and leg bases rather than along the abdomen.
What it is
Most stoneflies want cold, fast, well-oxygenated water, and they say so by being where it is. The nymph is a crawler: it creeps under and among stones rather than moving about in open water.
Stages
Nymph (5–70 mm (many western species large)) · Adult (6–75 mm; most 13–50 mm)
Water
river, stream, spring
Separates from Hellgrammite
This entry does not name Hellgrammite as a look-alike. That is a gap in the prose, not proof they cannot be confused.
Typical windows
Nymphs present (nymph) · Pre-emergence migration (nymph) · Adult activity (adult) · Small winter species (adult)
Patterns (correspondence)
Rubber-Legs Stonefly Nymph · Twenty-Incher · Stimulator · Little Black Stonefly
Limits
Habitat and emergence timing vary considerably by species.
